1. Field of the Invention
The present invention relates to a source driver for Co generating gray scale voltages supplied to source lines depending on data signals, a source line drive circuit using the source driver, and a matrix display device using the source driver and the source line drive circuit. Particularly, the invention relates to a source driver used for a display device, such as a liquid crystal display device, which is required to be driven by AC voltages because pixels constituting the display screen of the display device may be deteriorated or broken if DC voltages are applied thereto, and the invention also relates to a source line drive circuit using the source driver and to a display device comprising the source driver and the source line drive circuit.
2. Description of the Related Art
In recent years, active-matrix liquid crystal display devices capable of attaining fine display on a large screen have been developed increasingly. In the above-mentioned active-matrix liquid crystal display devices, a configuration wherein a thin-film transistor (TFT) array is formed by a thin-film technology on one of a pair of substrates holding a liquid crystal therebetween has been adopted widely.
FIG. 9 is a circuit diagram showing an example of an equivalent circuit of each pixel in a conventional active-matrix liquid crystal display device. Each pixel is provided corresponding to the intersection of a source line 4 and a gate line 5 disposed so as to be orthogonal to each other as shown in FIG. 9. A TFT formed by using amorphous silicon or the like for example is provided at each pixel, the gate line 5 is connected to the gate electrode of the TFT, and the source line 4 is connected to the source electrode of the TFT. A liquid crystal cell capacitance CLC, an auxiliary capacitance CS and a parasitic capacitance Cgd are connected as loads to the drain electrode of the TFT. The above-mentioned parasitic capacitance Cgd is generated by the capacitance coupling between the gate line 5 and the drain electrode used as a display electrode. The terminals of the liquid crystal cell capacitance CLC and the auxiliary capacitance CS, not connected to the drain electrode of the TFT, are connected to a common electrode (not shown) on an opposite substrate, and a common electrode voltage VCOM is applied to the terminals. In the above-mentioned configuration, a predetermined voltage depending on a data signal is written at the liquid crystal cell capacitance CLC and the auxiliary capacitance CS during a scanning period, thereby attaining predetermined gray scale display.
When an electric field having a constant direction is kept applied to a liquid crystal for a long time, the liquid crystal deteriorates because of its electrochemical property. For this reason, it is necessary to drive the liquid crystal so that the direction of the electric field to be applied to the liquid crystal is reversed periodically. In a dot inversion system, a gray scale voltage VX output from a source driver is reversed based on a polarity reversal signal REV so as to be centered with respect to the common electrode voltage VCOM and this alternating voltage drives a liquid crystal cell.
The liquid crystal cell voltage VLC generating at the liquid crystal capacitance CLC when the gray scale voltage VX is applied is a voltage difference between the common electrode voltage VCOM and the gray scale voltage VX supplied from the source line 4 via the source electrode and the drain electrode of the TFT, provided that the effect of the parasitic capacitance Cgd is ignored. In actual operation, however, it is impossible to ignore the parasitic capacitance Cgd.
The effect of the parasitic capacitance Cgd upon the drive of the pixel will be described below referring to FIG. 10. FIG. 10 shows the waveform of a scanning voltage VY supplied to the gate line 5, the waveform of the gray scale voltage VX output from the source driver, the waveform of the polarity reversal signal REV, the waveform of the common electrode voltage VCOM and the waveform of the liquid crystal cell voltage VLC generated by these voltages at the liquid crystal cell capacitance CLC. As shown in FIG. 10, when a selection pulse is applied to the gate electrode of the TFT via the gate line 5, the TFT is turned on. The gray scale voltage VX applied to the source line 4 is sent from the source electrode via the drain electrode to the liquid crystal cell capacitance CLC and the auxiliary capacitance CS used as the loads of the TFT. As a result, the liquid crystal cell voltage VLC rises in synchronization with the selection pulse. The voltage at the time when the selection pulse falls (hereinafter referred to as a final writing voltage) is retained by the liquid crystal cell capacitance CLC and the auxiliary capacitance CS. In reality, however, a level shift xcex94V occurs between the final writing voltage and the retaining voltage after the turning off of the TFT because of the effect of the redistribution of charges to the parasitic capacitance Cgd.
The level shift xcex94V acts to decrease the retaining voltage so that the retaining voltage becomes lower than the final writing voltage in the case where the liquid crystal cell voltage VLC is positive just as in a scanning period T1 shown in FIG. 10. In the case where the liquid crystal cell voltage VLC is negative just as in a scanning period T2, however, the level shift xcex94V acts to increase the retaining voltage so that the retaining voltage becomes higher than the final writing voltage.
As a result, the effective value of the liquid crystal cell voltage VLC in the scanning period T1 differs from that in the scanning period T2, whereby a DC voltage is applied to the liquid crystal, thereby deteriorating the liquid crystal. In addition, since the value of the positive voltage applied to the liquid crystal differs from the value of the negative voltage applied thereto, the luminance of the liquid crystal differs depending on the voltage value, thereby causing flicker in image display. To solve this problem, it has conventionally been proposed that the common electrode voltage VCOM should be shifted by the same amount as that of the level shift xcex94V so that the effective value of the positive liquid crystal cell voltage VLC is equal to the effective value of the negative liquid crystal cell voltage VLC.
The level shift xcex94V occurs because of the existence of the parasitic capacitance Cgd as described above. When the amplitude of the scanning voltage VY is VG the level shift xcex94V is represented by the following expression 1:
xcex94V=(Cgd/(Cgd+CLC+CS))xc2x7VGxe2x80x83xe2x80x83(1) 
When the cell gap is d, the area of the display electrode is A, the specific dielectric coefficient of the liquid crystal material is xcex5LC, and the dielectric coefficient of free space is xcex50, the liquid crystal cell capacitance CLC is represented by the following expression 2:
CLC=(xcex5LCxc2x7xcex50/d)xc2x7Axe2x80x83xe2x80x83(2) 
The specific dielectric coefficient xcex5LC of the liquid crystal material changes depending on the arrangement state of liquid crystal molecules, that is, depending on the liquid crystal cell voltage VLC. Therefore, the liquid crystal cell capacitance CLC is given as a function f1 of the liquid crystal cell voltage VLC and represented by the following expression 3. K1 is a constant.
CLC=K1xc2x7f1xe2x80x83xe2x80x83(3) 
Therefore, the level shift xcex94V is also given as a function f2 of the liquid crystal cell voltage VLC and represented by the following expression 4.
K2 is a constant.
xcex94V=K2xc2x7f2(VLC)xe2x80x83xe2x80x83(4) 
Furthermore, the light transmittance of the liquid crystal changes nonlinearly with respect to the magnitude of the liquid crystal cell voltage VLC. In other words, since the effective value of the liquid crystal cell voltage VLC differs at each gray scale level when attaining gray scale display, it is found that the magnitude of the level shift xcex94V at each gray scale level is not constant. Therefore, it is necessary to correct the level shift xcex94V at each gray scale level.
First, the general configuration of a conventional active-matrix liquid crystal display device will be described below. As shown in FIG. 11, the conventional active-matrix liquid crystal display device comprises a pixel array 1 having a plurality of pixels arranged in matrix, a liquid crystal panel having a plurality of source lines (not shown) and a plurality of gate lines (not shown) disposed orthogonal to one another, a source line drive circuit 8 for driving the source lines, and a gate driver 3 for driving the gate lines.
The source line drive circuit 8 is provided with a source driver 2 and a plurality of reference voltage generation circuits 9 (for positive and negative voltages) for supplying reference voltages to the source driver 2. The output voltage generation portion of the source driver 2 comprises a gray scale voltage generation circuit (not shown), a gray scale selection circuit (not shown) and an output buffer (not shown). The positive reference voltages and the negative reference voltages generated by the positive (High) reference voltage generation circuits and the negative (Low) reference voltage generation circuits respectively are supplied to the gray scale voltage generation circuit via the gray scale voltage input terminals of the source driver 2.
The gray scale voltage generation circuit is provided with a resistance-type voltage division circuit comprising a plurality of resistors connected in series. The voltage between the positive and negative reference voltages is equally divided by the resistance-type voltage division circuit to generate a plurality of output gray scale voltages. One of the generated a plurality of gray scale voltages is selected by the selection circuit depending on output gray scale data and output to the source line 4 of the liquid crystal panel via the output buffer.
At this time, the level shift xcex94V is present as described above. Therefore, it is necessary to carry out a correction (hereinafter referred to as the correction of the xcex94V characteristic). In order to ideally correct the level shift xcex94V characteristic with respect to the voltage applied to the liquid crystal, a proper gray scale voltage should be applied for each gray scale voltage. However, if all gray scale voltages are input to the source driver 2, the circuit is required to be very large in size, and this is not practical. For this reason, about five positive reference voltages and about five negative reference voltages are usually supplied to the reference voltage input terminals of the source driver 2. The voltage between reference voltages adjacent to each other is equally divided by the series resistors of the gray scale voltage generation circuit inside the source driver 2 in order to reduce the deviation of xcex94V.
The conventional source driver is provided with a plurality of reference voltage input terminals connected to the gray scale voltage generation circuit disposed inside the source driver, and the resistance value between the input terminals adjacent to each other is equally divided to generate still more gray scale voltages. In addition, the positive gray scale voltage generation series resistors are made symmetrical with the negative gray scale voltage generation series resistors inside the source driver. For this reason, in the case where the highest-level voltage and the lowest-level voltage are supplied only to the highest-level and lowest-level reference voltage input terminals respectively, the positive gray scale voltage and the negative gray scale voltage at each gray scale level are generated so as to be vertically symmetrical with each other. However, a level shift xcex94V being different at each gray scale level is present at the time of driving the liquid crystal as described above, and the xcex94V characteristic must be corrected. For this purpose, asymmetric voltage values in consideration of the xcex94V are usually supplied to the about five positive reference voltage input terminals and the about five negative reference voltage input terminals of the source driver as described above. The voltage between the voltages adjacent to each other is equally divided by the series resistors of the gray scale voltage generation circuit inside the source driver in order to reduce the deviation of xcex94V.
There are generally two reasons for the supply of a plurality of reference voltages from outside to the source driver. A first reason is to attain smooth gray scale display, and a second reason is to optimize the correction of the level shift xcex94V characteristic.
The first reason will be described below. The gray scale voltage generation circuit inside the source driver comprises series resistors divided equally, and voltages are usually supplied from outside so as to conform to the characteristic of an image. However, in the case where the number of input points is scarce, the luminance change between the reference inputs adjacent to each other becomes linear in the characteristic with respect to gray scale and luminance because of the equal division. For this reason, the luminance change does not become smooth as shown in the solid lines of FIG. 12. The solid lines of FIG. 12 show the characteristic with respect to gray scale and luminance for the conventional source driver Each plot point is a point wherein a gray scale voltage is input. FIG. 12 shows a case where five gray scale voltages are input from outside. The broken line of FIG. 12 shows an ideal characteristic with respect to gray scale and luminance on the assumption that all gray scale levels are displayed smoothly in the case of 64 gray scale levels. However, in the conventional case where the number of gray scale voltage input points is scarce, that is, about five, the luminance changes linearly as shown in the solid lines of FIG. 12, whereby it is impossible to obtain an ideal characteristic with respect to gray scale and luminance. This kind of technology for improving the characteristic with respect to gray scale and luminance by providing a plurality of reference points has been described in Japanese Unexamined Patent Publication JP-A 61-4374 (1986) for example.
Next, the second reason will be described below. FIG. 13 shows the center value (the average value of the positive and negative voltages) of the output voltage and the level shift xcex94V characteristic in the case of the conventional driver wherein the resistance values inside the source driver are equally divided between the reference voltages. The abscissa represents gray scale, and the ordinate represents voltage, The curve 32 of FIG. 13 indicates the level shift xcex94V characteristic at each gray scale voltage. The broken line 31 indicates the center value of the source driver generation voltage at the time when the voltage between the reference voltages is equally divided. If the broken line 31 coincides with the curve 32, no DC voltage is applied to the liquid crystal, and the liquid crystal is AC-driven properly. However, if the reference voltages are input sparsely as described above, a voltage generated by equal-division resistors is output as indicated by the broken line 31 at a gray scale level other than the gray scale levels wherein optimum reference voltages in consideration of the level shift xcex94V are input. Therefore, the xcex94V characteristic is not corrected sufficiently, and the output voltage deviates from the voltage based on the proper xcex94V characteristic by Va. If the amount of this deviation is large, AC-drive is not carried out optimally, and a DC voltage is applied to the liquid crystal, thereby not only causing the deterioration of the liquid crystal but also causing the problems of flicker and image persistence.
The above-mentioned two problems significantly lower the performance of the display device. In order to improve the quality of the display, numerous reference voltage input points become necessary. However, the number of the reference voltage input points is limited because of the size of the circuit and the like. Usually, about five points at most are provided for both the positive and the negative reference values. Even in this case, since the resistance value between the reference voltages is equally divided inside the source driver, the xcex94V characteristic cannot be corrected precisely as described above, and a DC voltage is applied to the liquid crystal. In addition, the change ratio in luminance ahead of and behind the reference voltage input point changes abruptly. Therefore, in the case where gray scale ramp display (display of an image linearly changing from white to black) is carried out, unnatural luminance change is recognized definitely.
Furthermore, in the conventional technology, in order to correct the xcex94V characteristic, a variable resistor for changing the potential of the common electrode is provided in a common electrode drive circuit. The resistance value of the variable resistor is adjusted so that flicker is reduced at any given gray scale points by carrying out visual checking or image recognition of a flicker evaluation pattern at each gray scale level, whereby the common electrode voltage VCOM is set close to a proper value.
However, in the conventional technology wherein the voltage between the external reference voltages is equally divided by the resistance-type division circuit inside the source driver, the voltages obtained by the correction of the level shift xcex94V do not completely conform to the proper voltages at all gray scale levels as shown in FIG. 13. Therefore, even if the common electrode potential VCOM is adjusted so that no flicker appears at a certain gray scale level, the positive liquid crystal cell voltage VLC and the negative liquid crystal cell voltage VLC have values different from each other at other gray scale levels, and flicker occurs at the voltages at those gray scale levels, thereby impairing the quality of display. In addition, the adjustment of the common electrode potential VCOM is very difficult and takes time.
Furthermore, Japanese Unexamined Patent Publication JP-A 7-92937 (1995) discloses a liquid crystal display device drive method capable of preventing an afterimage phenomenon while attaining multi-level gray scaling. In this method, a gray scale voltage generation circuit for supplying gray scale voltages to a source driver is provided outside the source driver, the addition voltage +V1 and the subtraction voltage xe2x88x92V1 of a maximum amplitude voltage VS and a reference voltage VC are alternately supplied by an alternating signal to both the end terminals of a resistance-type voltage division circuit formed in a gray scale voltage generation circuit to generate a plurality of gray scale voltages as shown in FIG. 14. Furthermore, an intermediate voltage VSSC supplied to the intermediate point of the resistance-type voltage division circuit is shifted from the reference voltage VC as shown in FIG. 15A and FIG. 15B, whereby asymmetrical positive and negative gray scale voltages are output, and the center value of each gray scale voltage is set optimally with respect to the voltage of the common electrode.
However, in the case where N-level gray scale display is carried out by the drive method disclosed by the above-mentioned publication, the resistance-type voltage division circuit in the gray scale voltage generation circuit is required to be divided by N resistors for both the positive and negative voltages in order to optimally set the center values of all gray scale values. This makes the circuit large in size and increases production cost and power consumption, thereby being impractical. More particularly, when 64-level gray scale display is carried out when the difference between the highest-level reference voltage and the lowest-level reference voltage is 10 V for example, a voltage accuracy of about 5 mV is required in the intermediate gray scale display region wherein the accuracy should be highest in order to attain accurate gray scale display. To attain this, a resistance value accuracy of 0.05% is required. It is thus necessary to use resistors having an accuracy far higher than the resistance accuracy (1%) of resistors usually used as discrete resistors outside the source driver. In other words, it is impractical to attain such a high accuracy by using discrete resistors. Furthermore, if a circuit requiring such a high voltage accuracy is formed by using discrete components outside the source driver and voltage division is carried out, the problem of unstable voltage division values because of external noise from a backlight for example occurs, whereby the accuracy of the voltage values is low, and accurate gray scale display cannot be attained.
An object of the invention is to provide a liquid crystal display device capable of attaining smooth gray scale display and greatly improved display quality, free from problems such as flicker, image persistence and the like.
The invention relates to a source driver for supplying gray scale voltages depending on data signals, to pixels required to be AC-driven, comprising a resistance-type voltage division circuit for generating gray scale voltages,
wherein positive-side (high level) voltage resistance division ratios and negative-side (low level) voltage resistance division ratios of the resistance-type voltage division circuit are set so as to be asymmetrical with one another depending on level shift characteristics.
In accordance with the invention, the plurality of positive-side voltage resistance division ratios and negative-side voltage resistance division ratios of the resistance-type voltage division circuit provided in the source driver to generate gray scale voltages are set so as to be asymmetrical with one another in consideration of the nonlinear characteristic of the level shift xcex94V because of the anisotropy of the dielectric coefficient of the liquid crystal. Therefore, the correction of the level shift xcex94V characteristic can be carried out at each gray scale level, whereby the positive-side liquid crystal cell voltage VLC can be made equal to the negative-side liquid crystal cell voltage VLC at each gray scale level. In other words, an unnecessary DC voltage is not applied to the molecules of the liquid crystal, whereby image persistence does not occur, the display problem of flicker and the like can be solved and the quality of display can be improved greatly. Furthermore, all gray scale voltages are corrected completely in consideration of the level shift xcex94V. Therefore, at the time of the visual adjustment of a common electrode voltage VCOM by using a flicker evaluation pattern at each gray scale level, by just adjusting the common electrode VCOM so that flicker disappears at a given gray scale level, it is possible to completely solve the display problems of flicker and the like at all the gray scale levels. For this reason, the adjustment of the common electrode voltage VCOM can be carried out very easily, with the result that the operation time is shortened.
Furthermore, the invention relates to a source driver for supplying gray scale voltages depending upon data signals, to pixels required to be AC-driven, comprising a resistance-type voltage division circuit for generating gray scale voltages, wherein resistance ratios of the resistance-type voltage division circuit are optimized depending on gray scale display characteristics. In particular, the resistance division ratios of the resistance-type voltage division circuit respectively constitute the ratios of the resistances of the series-connected resistors of the resistance-type voltage division circuit present between each adjacent pair thereof in order to generate each of the desired gray scale voltages, i.e., those resistance ratios required for generating the desired level shift compensated voltage divisions between a positive-side highest-level gray scale reference voltage and a negative-side lowest-level gray scale voltage by the resistors of the resistance-type voltage division circuit.
In accordance with the invention, the plurality of resistance division ratios of the resistance-type voltage division circuit provided in the source driver to generate gray scale voltages can be made highly accurate and can conform to a target xcex3 characteristic (gray scale display characteristic) by IC (integrated circuit) formation. Therefore, the source driver of the invention can output liquid crystal application voltages for attaining smooth gray scale display having an ideal xcex3 characteristic.
Furthermore, the invention relates to a source line drive circuit for supplying gray scale voltages depending on data signals, to pixels required to be AC-driven, comprising the above-mentioned source driver and a gray scale reference voltage generation circuit, wherein the source driver is provided with a plurality of input terminals, to which a plurality of input terminals are supplied gray scale reference voltages each having a different voltage level, and positive-side and negative-side gray scale voltages are generated based on the plurality of gray scale reference voltages.
In accordance with the source line drive circuit of the invention, the resistance division ratios of the resistance-type voltage division circuit for generating gray scale voltages are set as described above. Therefore, unlike the case of the conventional source line drive circuit, pixels can be driven optimally without supplying gray scale reference voltages having numerous levels. As a result, it is possible to eliminate a gray scale reference voltage generation circuit to be provided outside the source driver of the source line drive circuit. Therefore, the overall size of the source line drive circuit can be made smaller, the cost of components can be reduced, and lower power consumption can be attained.
Furthermore, the invention relates to a source line drive circuit for supplying gray scale voltages depending on data signals, to pixels required to be AC-driven, comprising the above-mentioned source driver, wherein the source driver is provided with two input terminals, to one of which input terminals is supplied a positive-side highest-level reference voltage and to the other of which input terminals is supplied a negative-side lowest-level reference voltage, and positive-side and negative-side gray scale voltages are generated based on the highest-level reference voltage and the lowest-level reference voltage.
In accordance with the invention, the source line drive circuit supplies the positive-side highest-level reference voltage and negative-side lowest-level reference voltage to the source driver. By using the reference voltages, the resistance-type voltage division circuit inside the source driver can generate all positive-side and negative-side gray scale voltages accurately and properly. It is thus not necessary to provide a gray scale reference voltage generation circuit outside the source driver. Therefore, the overall size of the source line drive circuit can be made smaller, the cost of components can be reduced, and lower power consumption can be attained.
Furthermore, the invention relates to an active-matrix liquid crystal display device comprising a plurality of pixels disposed in matrix, a plurality of data signal lines disposed corresponding to columns of the pixels, a plurality of scanning signal lines disposed corresponding to rows of the pixels, switching devices at the individual pixels, and the above-mentioned source line drive circuit for driving the data signal lines.
In accordance with the active-matrix liquid crystal display device of the invention, the positive-side voltage resistance division ratios and negative-side voltage resistance division ratios of the resistance-type voltage division circuit provided in the source driver to generate gray scale voltages are set so as to be asymmetrical with one another. Therefore, the level shift xcex94V being different depending on each gray scale voltage is reflected to the resistance division ratios of the resistance-type voltage division circuit inside the source driver to correct the gray scale voltages. For this reason, it is possible to obtain an active-matrix liquid crystal display device capable of solving the display problem of flicker and the like and having greatly improved quality of display.
Furthermore, it is possible to generate ideal, highly accurate gray scale voltages conforming to the target xcex3 characteristic without supplying numerous external gray scale reference voltages to the source driver, unlike the case of the conventional source line drive circuit. Therefore, the gray scale reference voltage generation circuit provided outside the source driver can be made smaller. Therefore, the overall size of the source line drive circuit can be made smaller, the cost of components can be reduced, and lower power consumption can be attained.